Causes of Severe Magnetite Formation During Underbalanced Drilling and Practical Mitigation Strategies

Abstract

While drilling underbalanced with potassium chloride (KCl) brine and injected nitrogen generated by a membrane filter, black precipitate and heavy scale deposits on the drillstring were observed. The brine was treated with phosphate ester and an amine-based inhibitor at the optimum pH range to mitigate corrosion. A robust onsite monitoring program of the fluids' properties and the inhibitor concentration was carried out to help ensure proper control of corrosion during the underbalanced drilling (UBD) operations Analysis of scale-like deposits scraped off the drillstring was predominantly magnetite with trace amounts of siderite and KCl. Little to no scale was seen on the drillstring below the nitrogen injection point indicating that the injected gasses seemed to be a contributor to the issue.Examination of the drillstring surfaces beneath these scales and API corrosion rings did not exhibit metal loss or pitting indicating these scales were not consistent with corrosion. Further, scale formation continued to be observed as drilling continued under nil oxygen conditions when cryogenic nitrogen was employed and oxygen levels of less than 1% were maintained. Laboratory and field engineers were deployed to collect samples of the brine used for drilling, the scale deposits, solids recovered at the shale shaker screens, and the fluid returns from the wellbore. The precipitate was mostly amorphous iron oxide with magnetite (i.e., combination of Fe (II) and Fe (III) oxides that is ferrimagnetic in nature). This paper will present the field and lab data generated which helped in discovering the root cause of the problem and developing practical solutions. IntroductionField and laboratory investigation was undertaken to find the root cause of scaling of conventional drill pipe during UBD using a 2% KCl brine. Nitrogen generated by membrane filter was initially utilized to establish underbalanced conditions allowing influx from the drilled formation (i.e., hydrocarbons and water). Build up of a black precipitate or scale was observed on the outside of the pipe during a trip out of the hole. At first, corrosion was suspected and steps were taken to minimize the effects of corrosion. Cryonitrogen and oxygen scavengers were applied along with the corrosion inhibitors, but this treatment did not stop the precipitate formation. Underbalanced drilling specialists and laboratory scientists were sent to observe and analyze the situation on-sight and help determine the underlying cause of the precipitation phenomena and its source (i.e., scaling or corrosion).In order to fully recognize the concepts presented in this paper and provide the appropriate details, a brief overview of corrosion and its causes is provided. Corrosion of metals is a worldwide problem faced by the petroleum industry that also impacts many other facets of everyday living. The effects of corrosion costs billions of dollars in repairs each year not counting the costs of corrosion prevention (Muzyczko 1978). Developing a solution for such a large scale issue req...